1. Field of the Invention
The present invention relates to small internal combustion engines of the type used in a variety of applications, such as lawnmowers, generators, pumps, tillers, pressure washers and other lawn and garden implements, or in small utility vehicles such as riding lawnmowers, lawn tractors, and the like, as well as in sport vehicles.
2. Description of the Related Art
Generally, the intake and exhaust valves of small internal combustion engines may be actuated directly by a camshaft located in the cylinder head, or may be actuated indirectly through the use of rocker arms, tappets, or other similar means. For example, in many existing L-head and overhead valve (“OHV”) engines, the crankshaft drives a camshaft which is located within the crankcase and is disposed parallel to the crankshaft, and lobes on the camshaft actuate tappets, lifters, push rods and/or rocker arms to open and close the valves. In overhead cam (“OHC”), engines, a camshaft located in the cylinder head of the engine is driven from the crankshaft, and includes lobes thereon which directly actuate intake and exhaust valves.
At engine cranking speeds during engine starting, the intake and exhaust valves are both closed as the piston rises in its compression stroke toward its top dead center position, and substantial pressure is built up in the combustion chamber which resists movement of the piston toward its top dead center position. This pressure must be overcome to crank the engine for starting, and typically requires a substantial amount of force to be exerted by the operator, such as by pulling on the rope of a recoil starter. Therefore, small internal combustion engines typically include a type of compression release mechanism to aid in engine starting.
Also, at engine cranking speeds during engine starting, the intake and exhaust valves are both closed as the piston descends in its expansion stroke toward its bottom dead center position. During this stroke, the piston descends against a vacuum which is produced within the combustion chamber of the engine, thereby creating a vacuum force which resists downward movement of the piston and which must be overcome by the operator to start the engine. Therefore, small internal combustion engines may also include a type of combustion chamber venting arrangement, or “vacuum release” mechanism to aid in engine starting.
Compression release mechanisms for small internal combustion engines are usually operable at cranking speeds to prevent the exhaust or intake valve from fully closing as the piston approaches its top dead center position, thereby allowing venting of pressure from the combustion chamber. Vacuum release mechanisms for small internal combustion engines are also operable at cranking speeds to prevent the exhaust or intake valve from fully closing as the piston descends from its top dead center position, thereby allowing venting of air into the combustion chamber. In this manner, cranking of the engine is much easier and requires less force to be exerted by the operator. When the engine reaches a predetermined speed after starting, the compression and/or vacuum release mechanism is automatically rendered inoperative, such that the exhaust or intake valve fully seats or closes as the piston approaches and/or descends from its top dead center position to allow combustion to proceed in a normal manner.
Many known compression release mechanisms and vacuum release mechanisms include a large number of individual, moving parts, and can be somewhat mechanically complex. Although many known compression release mechanisms and vacuum release mechanisms operate well, the number of parts from which these mechanisms are made increases the cost and difficulty of assembling such mechanisms.
What is needed is a compression and/or vacuum release mechanism for small internal combustion engines which includes a relatively few number of parts, is durable, and which is compact in construction.
Additionally, it is known that for many internal combustion engines, the optimum valve operating characteristics may vary between low engine speeds and high engine speeds. Some internal combustion engines include low speed cams having a first cam profile for actuating the intake and exhaust valves at low engine speeds, and high speed cams having a second cam profile for actuating the intake and exhaust valves at high engine speeds. These engines incorporate cam switching mechanisms in which the low speed cams are used at low engine speeds, and the high speed cams are used high engine speeds. Typically, however, cam switching mechanisms are mechanically very complex and expensive to manufacture, such that same are usually not used in small internal combustion engines.
Thus, a further need is for a cam switching mechanism for small internal combustion engines which is an improvement over the foregoing.
Additionally, some known internal combustion engines incorporate a low oil level warning and/or a low oil shut down feature which is responsive to the oil level in the engine crankcase. When the oil level falls below a level which is necessary to adequately lubricate the moving parts of the engine, such that damage to the engine could potentially occur, a low oil level warning is signaled to the operator or the engine is automatically shut down to prevent damage to the engine. Typical low oil level warning and/or low oil shutdown mechanisms rely upon direct oil measurement devices, such as float valves or electronic sensors disposed in the crankcase, which add cost to the engine.
What is needed is a low oil shutdown feature for small internal combustion engines which is an improvement over the foregoing.